1. Field of the Invention
The present invention relates generally to an optical pickup device for used in an optical information recording/reproducing apparatus.
2. Description of the Related Art
The optical pickup devices are roughly classified into two sorts that one employs a polarizing optics system and the other employs a non-polarizing optics system.
For example, FIG. 1 shows an optical pickup device with a polarizing optics system using an astigmatism method.
A semiconductor laser 1 irradiates a laser beam which is a linearly polarized light to a collimator lens 2. The collimator lens 2 transforms this laser beam into a parallel laser beam which travels through a polarization beam splitter 3 and a quarter-wave plate 3a. The quarter-wave plate 3a converts the laser beam linearly polarized light into a circularly polarized light. An objective lens 4 converges the circularly polarized light beam toward an optical disc 5 to form a light spot onto a pit train on an information recording surface of the optical disc 5.
An optical disc 5 reflects the circularly polarized light back to the objective lens 4. The objective lens 4 converges the reflected light to the quarter-wave plate 3a in which the circularly polarized light is converted to a linearly polarized light (which has a plane of vibration perpendicular to that of the original linearly polarized light). This returned linearly polarized light travels through the polarization beam splitter 3. The polarizing film of the polarization beam splitter 3 reflects and directs the linearly polarized light to a detecting lens 7. The detecting lens 7 converges the linearly polarized light to a cylindrical lens 8, serving as an astigmatism generating element, to form a light spot near the center of a light receiving surface of a quadrant photodetector 9 having four light receiving surface areas (elements) divided by two orthogonal line segments.
The quadrant photodetector 9 opto-electrically transduces the light spot irradiated to the four light receiving surface areas into respective electric signals to supply them a demodulating circuit 12a and an error detecting circuit 12b. The demodulating circuit 12a generates an electric data signal on the basis of the supplied signals. The error detecting circuit 12b generates a focus error signal and a tracking error signal based on the electric signals supplied from the quadrant photodetector 9 and supplies these error signals to an actuator driver circuit 13. The actuator driver circuit 13 supplies a corresponding driving signals to an actuator 15. The actuator 15 drives and servo-controls the objective lens 4 in response to the driving signal.
On the other hand, FIG. 2 shows the other optical pickup device in which the non-polarizing optics having a small polarization i.e., a small difference between the S- and P-polarized light components such as a half mirror 3b (in which a transmittance T is nearly equal to a reflectivity R) is employed instead of the optical isolator having a combination of both the polarization beam splitter and the quarter-wave plate as shown in FIG. 1.
A polarization beam splitter included in the polarizing optics system has a polarizing film comprising a multilayer dielectric layer which has an optical characteristics such that a transmittance for a P-polarized light component T.sub.P is nearly equal to 1, a transmittance a for S-polarized light component T.sub.S is nearly equal to 0, a reflectivity for a P-polarized light component R.sub.P is nearly equal to 0, and a reflectivity for a S-polarized light component R.sub.S is nearly equal to 1. In addition, a quarter-wave plate comprising a uniaxial crystal is disposed in such a manner that the principal plane of the quarter-wave plate is in an angle 45 degree with respect to a plane of vibration of the P-polarized light entering thereinto. This combination of the polarization beam splitter and the quarter-wave plate is an optical isolator which changes the polarizing directions of light in the forward and back optical paths. Therefore, the polarization beam splitter divides the P-polarized light in the forward optical path and the S-polarized light in the back optical path. This polarizing optics system has an advantageous effect of a high optical utility efficiency in comparison with a non-polarizing optics system.
In a two focus pickup device used in a compatible player for a digital video disc (DVD) and compact disc (CD), such a polarizing optics system with a high optical utility efficiency is employed to compensate a reduction of diffraction light intensity caused by the usage of hologram which separates the zero order diffraction light for DVD and the primary order diffraction light for CD.
Whereas, a transparent substrate for an optical disc such as CD is generally made of a thermoplastic resin such as polycarbonate. In this case, the thermoplastic resin is injection-molded into a die of a master disc or stamper to form the substrate. Since it is difficult to homogeneously expand the resin in the stamper, unevenness of thickness appears in the resultant transparent substrate after the manufacturing process. Such an unevenness of thickness of the substrate causes a double refraction with respect to the light beam irradiated to the optical disc.
In the pickup device, a reflected light returned from an optical disc with a double refraction through the quarter-wave plate does not have mainly an S-polarized light component. There is a problem that the optical intensity on the photodetector is reduced to a level lower than that of a non-polarizing optics system although the polarizing optics system is employed.